1. Field of the Invention
The present invention relates to improvements in an optical scanning device for scanning a subject medium.
2. Description of the Related Art
In recent years, with development of digital techniques and light sources, numerous high performance printing equipment and the like have been developed, wherein a scanning beam modulated according to image signals scans a subject matter, such as photographic pictures and printed matter (which is hereafter referred to as a scanning subject medium), that is sensitive to light to record an image on a paper. In such a printing equipment, the utilization is made of an optical scanning device to scan a scanning subject medium. This optical scanning device is, for typical example, configured such that a laser beam from a laser diode is repeatedly reflected and deflected by a rotary polygon mirror and further directed as a scanning beam to a subject medium through an f-θ lens.
In this type of optical scanning device, the polygon mirror that rotates at a speed sufficiently high causes a strong current of air therearound. In the case where the polygon mirror is received in a somewhat isolated chamber, the strong current of air hits against walls of the isolated chamber, or even in the case where the polygon mirror is placed in an open space, the current of air strikes parts laid around the polygon mirror, the current of air gets disturbed with an adverse effect of causing irregular rotation, or fluctuations of rotation, of the polygon mirror, which leads to unevenness of scanning on a subject medium, and hence inaccurate scanning. Although it is though to dispose the polygon mirror in a spacious place in the housing on the ground of this problem, it is undesirable in light of miniaturization of the optical scanning device.
A polygon mirror and an electric motor are usually previously assembled and attached to a base board 18 as one whole of polygon mirror unit for easy installation and removal and/or easy replacement upon an occurrence of break-down. The polygon mirror unit is usually installed to a precise mount provided within a body housing with an intention to place and adjust the polygon mirror in position accurately relative to a scanning beam projection optical system and a scanning optical system. Fluctuations of rotation of the polygon mirror had been a great problem in the optical scanning device of this type. For the purpose of providing a brief background that will enhance an understanding of the behavior of a current of air caused by a polygon mirror in an isolated chamber, reference is made to FIG. 7.
As shown in FIG. 7, a polygon mirror unit 300 which comprises an electric motor 319, a polygon mirror 317 integrated with an output shaft (not shown) of the electric motor 319, and a base board 318 on which the electric motor 319 secured is installed to a mount 310 provided within a dust free chamber 320. The mount 310 comprises U-channel support frames 310a arranged with an separation therebetween. The base board 318 at its four corners is secured to the U-channel support frames 310a by fastening bolts 310b. There are provided spaces below the polygon mirror unit 300, in particular, the base board 318.
In the mount thus constructed, when the polygon mirror 317 rotates at a high speed and causes a current of air within the dust free chamber 320, the current of air hits against walls of the dust proof chamber 320, as a result of which a turbulent air flow is generated. Under an influence of high speed rotation of the polygon mirror 317, the current of air partly easily flows into the spaced formed below the base board 318, so as to generates turbulent air flows with an adverse effect of distorting the base board 318 of the polygon mirror unit 300. In consequence, the polygon mirror 317 causes fluctuations of rotation, which is always undesirable for precise and stable scanning operation of the optical scanning device. Shown by reference characters 23, 24 and 25 in FIG. 7 are optical elements forming part of an f-θ lens system.